RE: Air Change Rates

While I have never seen an ASHRAE recommendation on this, the Wisconsin building code, at one time, required 6 air changes per hour for a space that was not provided with mechanical cooling. It's a rule of thumb that I've kept in mind for getting a rough idea of air flow requirements, but that won't take you very far with code compliance or occupant comfort.

RE: Air Change Rates

How many cubic feet per minute (CFM) of outdoor air do you need in order to keep the interior air fresh?

ASHRAE 62.2 method: CFM = (0.01 x floor area ft2) + [7.5 x (# of bedrooms +1)]This protocol uses the ANSI Z765 floor area calc method (exterior dimensions), not treated floor area (TFA, interior dimensions) used by PHPP. The method assumes the building is leaky and some of the necessary outdoor air is supplied by leaks. The infiltration credit is 0.02 CFM/ft2. Designing for an airtight (passivehouse) building, you would need to add an additional 0.02 CFM/ft2 to the required total to be supplied by mechanical ventilation.

DIN 1946 Part 6 method (this is the Euro/German method embedded in PHPP). Essentially 18 CFM per person. Example: 6 people x 18 CFM = 108 CFM continuous. But the prescriptive exhaust air volume can trump this prescriptive supply volume, if there are many kitchens and baths. DIN 1946-6 also has a great method for assigning fractions of the total outdoor air volume to each room. ASHRAE has methods for this, but they are written for non-residential occupancies. DIN 1946-6 is written specifically for residential ventilation.